O'Brien (O'Brian), John Adrian; Taken on strength in 15th Field Regiment RCA with former service in Seaforth Highlanders on 3 May 1951, struck off strength on 16 Apr 1953, taken on strength on 28 Jan 1954, struck off strength on transfer to Seaforth of Canada on 6 Dec 1954.

O'Brien, Terence C; Taken on strength Canadian Officers Training Corps in 1952, Second Lieutenant in 1954, taken on strength from Canadian Officers Training Corps on 1 Oct 1955, promoted Lieutenant on 19 Jul 1956, Captain, deceased 22 May 2015.

Oceguera, Ian M; Taken on strength in 15th Field Artillery Regiment RCA, Second Lieutenant from 2013 to 2014, Lieutenant in 2017.​

Olinek, Jacquelynn Denise; Taken on strength as a Gunner in 15th Field Artillery Regiment RCA on 22 Apr 1997, qualified QL2 General Military Training on 3 Aug 1997, qualified QL3 Artillery on 22 Aug 1997, released on 28 Apr 1998.

O'Malley, Samuel Rodger Charles; Taken on strength from General List to 15th Coast Regiment RCA as Lieutenant on 1 Apr 1946 to 1950, awarded the Efficiency Medal on 15 Feb 1950, struck off strength on transfer to Supplementary Reserve on 25 Oct 1951.EM

Ormston, James Joseph; Taken on strength as a Musician in 5 (BC) Field Battery RCA on 27 May 1982, qualified TQ1 Musician on 17 Jun 1982, qualified TQ2 Musician on 28 Aug 1982, qualified General Military Training on 28 Aug 1982, promoted Bombardier on 27 May 1983, released on 8 Feb 1984, taken on strength in Royal Westminster Regiment on 2 Jul 1984, transferred to 15th Field Artillery Regiment RCA on 22 May 1985, transferred to Supplementary List on 9 Jan 1986.

​Orr, RT; Private in 158 Battery in 1969, Sergeant.

Orre, Glen Hugo; Taken on strength with 31 Battery 15 Field Regiment RCA on 22 Jan 1948, appointed Lance Bombardier on 23 Sep 1948, struck off strength on 10 May 1951.

Orsolan, Frank Luigi; Taken on strength as a Gunner in 15th Field Artillery Regiment RCA on 25 Mar 1982, released on 30 Apr 1983.

Osborne, Dennis Joel; Taken on strength 15 Field Regiment RCA on 13 May 1954, struck off strength on release on 13 Jan 1955.